Use a Fixed Timestep

[rj00a]

Describe the problem related to your feature request.

Valence currently uses the ScheduleRunnerPlugin to run each tick with a fixed delay. Problems arise because ScheduleRunnerPlugin does not account for thread::sleep inaccuracies. This is especially apparent on platforms like Windows where the timer resolution is very imprecise (16ms) by default. Regardless, this is probably not the intended use for ScheduleRunnerPlugin.

The obvious solution is to use bevy_time and CoreSet::FixedUpdate. However, there are two problems:

1. The default base set for the Main schedule is CoreSet::Update. It's far too easy to accidentally add systems to the wrong set when .in_base_set(CoreSet::FixedUpdate) is forgotten.
2. There are no standard PreFixedUpdate, PostFixedUpdate, sets. Plugins and Valence internally need to run code before and after user code.

Bevy 0.11 will fix (1) with the "schedule-first" redesign. But there is still no solution for (2).

What solution would you like?

When Bevy 0.11 is released, create a TickLoop schedule and use the facilities in bevy_time to run our own TickFirst, PreTick, Tick, PostTick, TickLast schedules in that order. User code goes in Tick.

What alternative(s) have you considered?

We could modify the app's runner to match the behavior of CoreSet::FixedUpdate. But that will cause problems for graphical plugins that will likely replace the runner by using WinitPlugin.

Additional context